This invention relates to medical/dental handpieces, and in particular to a chuck for the handpiece which releasably holds and drives a cutting, polishing, or grinding tool. In its broader aspects the invention may also find application in other chucking systems which hold and rotate shafts.
Dental handpieces include both high speed turbine types (typically 200,000 to 400,000 rpm) having an air motor and gear-driven lower speed types (typically, 1,000 to 15,000 rpm) having an air motor or an electrical motor. Both have long employed collect chucks for releasably holding dental burs which cut, grind and polish teeth. Similar burs are used in surgery for performing these functions on bone. Because the shafts of dental tools are quite small and quite closely toleranced (shaft diameters between 0.0626" and 0.0630"), and because the demands of dental and surgical procedures require the utmost performance, collet chucks are high precision devices requiring a great deal of care and expense for their manufacture. The collet chucks include a tube which aligns the bur and radially movable fingers which hold the shaft of the bur both axially and rotationally. The tube, fingers, and the operating mechanism for the fingers must be formed precisely.
The chucks commonly used in dental (or surgical) handpieces generally fall into two groups: those including a positive screw thread adjustment for tightening and loosening the fingers of the chuck, and those which use a spring for tightening the chuck and some mechanism for loosening the spring. Both types of chucks have drawbacks. The threaded type requires a separate wrench which is difficult to handle, must be separately sterilized, and must be accounted for. When used with a turbine handpiece, a mechanism must be provided in the wrench or otherwise for holding the driven rotor of the turbine from rotation while turning the threaded member. The spring-biased type requires a means for opening the fingers against the spring three; a spring force great enough to hold the bur shaft reliably requires exerting a great deal of effort to open the fingers. See for example U.S. Pat. Nos. 3,637,050 to Hoffmeister, 4,874,314 to Fleer et al., 5,040,980 to Heil, and 5,090,906 to Pernot. The first two patents utilize a push button to open the chuck, and the latter two patents utilize lever arms to reduce the effort required to open the chuck. The problems with spring-biased collet chucks are described in some detail in Heil, U.S. Pat. No. 5,040,980. As further described in that patent, the force required to pull the bur shaft from the chuck when the bur is not turning is not necessarily a good predictor of how well the chuck will hold in actual use of the bur on tooth or bone. The problems are further compounded when the bur is used on disparate materials such as amalgam or the thin metal shell making up dental crowns, which can grab the bur.
Alternatives to the collet chuck have been proposed for dental angles. Examples are the expansible polyethylene sleeve shown in U.S. Pat. No. 3,324,553 to Borden, and the coil spring shown in U.S. Pat. No. 4,021,918 to Bailey. None of the alternatives, however, has provided the required precision and resistance to slippage and pull-out.